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Flashcards in Bacteriology Deck (10):

How is MIC tested?

least amount of antibiotics required to inhibit growth

Gradient antibiotic strips on agar - look for crossover of bacteria on the strip

Discs impregnated with different conc of antibiotic. Measure clerance zone guidelines have zone sizes on how this translated to MIC


What kind of investigations may be performed?

culture e.g. agar or a broth
Molecular techniques


What are the complications of starting patients on antibiotics before sending off samples?

The antibiotics can interfere with the test results - potential for false negatives as the dose administered may not have a clinical effect but it might have an effect at the cellular level on the agar

In meningitis or sepsis you administer anyway


What kind of samples might be used?

stool samples, mouth & vaginal swabs, taking blood


Describe susceptibility testing

Still use agar plates and MICs, takes about 18 hours


Describe how blood cultures are run

Different bottles for collection
pH indicator on bottom of bottle changes colour as pH lowered from bacteria producing CO2
takes about 18 hours


What kind of microbiological tests might be carried out?

Gram staining - double membrane are negative, single membrane are positive (peptidoglycan retains purple dye)

Microscopy - look at shape and organisation (clumping or chains)

Coagulase e.g. staf aureus is positive but other stafs are negative


What are the limitaions of microbiological investiagtions

slow - some e.g. TB can take 6 weeks to grow

MIC tests difficult to interpret


Explain the results of MIC

3 types of result resistant intermediate sensitive

guideline values for MIC enables the lab to say whether its sensitive or resistant – resistant because not enough of the drug will be able to reach the organisms. Guidelines have cross over values and zone sizes


What are the limitations of PCR?

-need to know what looking for to design primers
-potentially hundreds of primers needed as many genes confer resistance
-PCR less sensitive than culture